The extent to which basal ganglia disorders of postural control are associated with abnormal central processing of somatosensory information for an internal representation of body position and motion (kinesthesia) is unknown. This project seeks to determine how kinesthetic information is used for postural control in healthy elderly subjects and subjects with Parkinson's disease (PD), a basal ganglia disorder. Both subjective measures (psychophysical) and objective measures (surface reactive forces, kinematic body CoM displacement, and muscle activation) will be used to quantify disorders of kinesthetic integration for postural control in subjects with PD compared to age-matched control subjects. Standing and sitting postural control will be examined because of stance and axial postural problems associated with PD. All studies will test subjects with PD in the ON and OFF states to examine the effects of levodopa replacement on kinesthetic control of posture. The specific aims address 3 roles of kinesthetic information for the control of posture: Aim I. Determine how PD affects perception of distal and proximal joint position sense and perception of postural verticality. Exp. 1 will test the hypothesis that the basal ganglia are important for conscious ankle and torso kinesthesia in stance posture. Exp. 2 will test the hypothesis that the basal ganglia integrate vestibular and somatosensory information to provide an internal representation of postural vertical. Aim II. Determine whether PD impairs use of somatosensory graviception as a postural reference. Exps. 3 and 4 will test the hypothesis that PD is associated with impaired use of slow, graviceptive somatosensory information, which normally provides a reference or 'set' point for postural orientation to vertical. Aim III. Define how PD impairs movement-related kinesthetic information for compensatory stepping. Exp 5 and 6 will test the hypothesis that impaired kinesthesia from the legs and falling body results in increased dependence on vision for accurate compensatory stepping to recover equilbrium. Effective rehabilitation of balance disorders requires a better understanding of how kinesthetic information is used for joint alignment, postural verticality, postural orientation, and control of postural equilibrium responses.